Danny Sullivan also wrote an article (sub req) about how some large advertisers get additional SEO support from search engines.

Google is known to tell some large advertisers that it is OK to do things that are against their official webmaster guidelines.

If you selectively boost some sites it has the same net effect of manually penalizing or filtering others, which goes counter to that "democratic nature of the web" "we don't manually..." "don't be evil b/s."